I agree he has nothing to gain, but he shouldn't be in it for what he can gain. Allow someone fit for purpose to do it, and stick to the journalism (£5000 per column?).
I think it's right and fair that the other leaders have faced an aggressive line of questioning, it's a proxy job interview process.
There's obviously been a Tory calculation behind the scenes that looking a coward is less harmful than trying to defend the last nine years, but I'm not sure they've got their workings right there – if they learned their lessons from last time, the gap started closing in the polls around the time May shied away from the TV debate so she could sit at home and work on her Brexit plan instead (and after the repeated failure of it how does that excuse look now). From a tactical point of view all that Johnson needs to do is face the same format and come across better, is he not confident he can do that? Whoever wins will have years of rigorous negotiations ahead of them, a week before this election is IMHO the worst possible time for a potential PM to look like a chicken.